E-selectin, which has also been called ELAM-1 for endothelial leukocyte adhesion molecule-1 and LECAM-2 for lectin cell adhesion molecule, is a glycoprotein that is found on the surface of endothelial cells, the cells that line the interior wall of capillaries. E-selectin recognizes and binds to the carbohydrate sialyl-Lewis.sup.x (sLe.sup.x), which is present on the surface of certain white blood cells. E-selectin helps white blood cells recognize and adhere to the capillary wall in areas where the tissue surrounding the capillary has been infected or damaged. E-selectin is actually one of three selectins now known. The other two are L-selectin and P-selectin. P-selectin is expressed on inflamed endothelium and platelets, and has much structural similarity to E-selectin and can also recognize sialyl-Lewis.sup.x. The structure of sialyl-Lewis.sup.x and sialyl-Lewis.sup.a (sLe.sup.a) are shown in formulas I.sub.a and I.sub.b below: ##STR2##
When a tissue has been invaded by a microorganism or has been damaged, white blood cells, also called leukocytes, play a major role in the inflammatory response. One of the most important aspects of the inflammatory response involves the cell adhesion event. Generally, white blood cells are found circulating through the bloodstream. However, when a tissue is infected or becomes damaged, the white blood cells must be able to recognize the invaded or damaged tissue and be able to bind to the wall of the capillary near the affected tissue and diffuse through the capillary into the affected tissue. E-selectin helps two particular types of white blood cells recognize the affected sites and bind to the capillary wall so that these white blood cells may diffuse into the affected tissue.
There are three main types of white blood cells: granulocytes, monocytes and lymphocytes. Of these categories, E-selectin recognizes sLe.sup.x presented as a glycoprotein or glycolipid on the surface of monocytes and neutrophils. Neutrophils are a subclass of granulocytes that phagocytose and destroy small organisms, especially bacteria. Monocytes, after leaving the bloodstream through the wall of a capillary, mature into macrophages that phagocytose and digest invading microorganisms, foreign bodies and senescent cells.
Monocytes and neutrophils are able to recognize the site where tissue has been damaged by binding to E-selectin, which is produced on the surface of the endothelial cells lining capillaries when the tissue surrounding a capillary has been infected or damaged. Typically, the production of E-selectins and P-selectins are increased when the tissue adjacent a capillary is affected. P-selectin is present constitutively in storage granules from which it can be rapidly mobilized to the cell surface after the endothelium has been activated. In contrast, E-selectin requires de novo RNA and protein synthesis, and peak expression does not occur until about 4-6 hours after activation, and declines to basal levels after about 24-48 hours. White blood cells recognize affected areas because sLe.sup.x moieties present on the surface of the white blood cells bind to E-selectin and P-selectin. This binding slows the flow of white blood cells through the bloodstream, since it mediates the rolling of leukocytes along the activated endothelium prior to integrin mediated attachment and migration, and helps to localize white blood cells in areas of injury or infection.
While white blood cell migration to the site of injury helps fight infection and destroy foreign material, in many instances this migration can get out of control, with white blood cells flooding to the scene, causing widespread tissue damage. Compounds capable of blocking this process, therefore, may be beneficial as therapeutic agents. Thus, it would be useful to develop inhibitors that would prevent the binding of white blood cells to E-selectin or P-selectin. For example, some of the diseases that might be treated by the inhibition of selectin binding to sLe.sup.x include, but are not limited to, ARDS, Crohn's disease, septic shock, traumatic shock, multi-organ failure, autoimmune diseases, asthma, inflammatory bowel disease, psoriasis, rheumatoid arthritis and reperfusion injury that occurs following heart attacks, strokes and organ transplants. In addition to being found on some white blood cells, sLe.sup.a, a closely related regiochemical isomer of sLe.sup.x, is found on various cancer cells, including lung and colon cancer cells. It has been suggested that cell adhesion involving sLe.sup.x may be involved in the metastasis of certain cancers.